The present invention relates to an operating method for a discharge lamp which is designed for dielectrically impeded discharges. For this purpose, the discharge lamp has a discharge vessel filled with a discharge medium, and at least one anode and at least one cathode. A dielectric layer is provided at least between the anode and the discharge medium, in order to produce dielectrically impeded discharges.
The terms anode and cathode are not to be understood in this application such that the invention is limited to unipolar operation. In the bipolar case, there is, at least electrically, no difference between anodes and cathodes, and so the statements for one of the two electrode groups then hold for all electrodes.
As promising fields of application for the discharge lamps considered here, mention may be made by way of example of the backlighting of flat display screen systems, or the backlighting of signal devices and signal lamps themselves. Reference is made in a supplementary fashion regarding the two last-named points to the disclosure content, hereby referred to, of EP-A-0 926 705. Furthermore, this invention is also suitable for lamps such as the copier lamp, represented in DE-A-197 18 395, with internal electrodes, and to the linear lamp, described in German application 198 17 475.6, with external electrodes. The disclosure content of the cited applications is respectively referred to hereby.
Because of the fact that discharge lamps for dielectrically impeded discharges can be designed in a very large multiplicity of the most varied sizes and geometries and, moreover, avoid the typical disadvantages of classic discharge lamps with mercury-containing filling in conjunction with a relatively high efficiency, it is expected that such discharge lamps will be used increasingly both with regard to their quantitative spread and with regard to their fields of use.
Reference is made to the following documents from the prior art:
DE 196 36 965 A1 exhibits discharge lamps for dielectrically impeded discharges which consequently exhibit a dielectric layer between at least the anode and the discharge medium. In accordance with this document, defined attachment points for individual discharges are created by localized field forcings. The homogeneity of the power distribution is intended thereby to be improved both in regard to time and in regard to space.
DE 197 11 893 A1 largely corresponds to the document just cited, and takes the teaching thereof further by using a denser arrangement of the attachment points in the edge region of the lamp or, alternatively, by increasing the current density through individual discharges burning there to counteract edge darkening by widening the anodes.
DE 41 40 497 C2 exhibits an ultraviolet high-power radiator with dielectrically impeded discharges in which the electric power converted in the edge region is increased by varying the discharge spacing or the dielectric capacitance in order to improve the homogeneity of the UV emission.
DE 42 22 130 A1 is concerned within the framework of dielectrically impeded discharges with the starting aid function of local field distortion structures, for example quartz drops melted onto discharge vessel walls, or dents or humps in the walls.
U.S. Pat. No. 5,760,541 describes a discharge lamp with strip-shaped electrodes whose geometric shape leads to a field modulation in the discharge lamp owing to sinusoidal edges, cutouts and other possibilities. The aim thereby is to eliminate temporal fluctuations in a bright/dark distribution in the discharge lamp in order to permit a temporally constant spatial correction of these heterogeneities for the benefit of applications in scanning devices for transparent media.
DE 196 28 770 relates to measures for optimizing the power output of a traveling-wave tube amplifier element at transponder level for satellite applications, in order to stabilize the output power of the overall amplifier system with regard to changes in the operating point, ageing, frequency changes, temperature fluctuations etc.
GB 2 139 416 describes the spatial modulation of the emission of radiation from an electron irradiation device by means of specific spatial arrangements of permanent magnets and magnetic materials.
U.S. Pat. No. 4,584,501 describes a discharge display in which various discharge paths are switched by mechanically actuated flaps, and optical effects are produced by multiple reflections by using semipermeable mirrors.
DE 198 17 479, published after the priority date, relates to the division of the electrode arrangement in a silent discharge lamp into different groups, which can be operated separately.
DE 43 11 197 describes the pulsed operating method, which is essential for the discharge lamps considered here, and the coordination of parameters in order to produce a specific type of discharge.
This invention is based on the technical problem of providing a further contribution to widening and improving the possibilities of use of discharge lamps for dielectrically impeded discharges.
This problem is solved according to the invention by means of an operating method for a discharge lamp having a discharge vessel, containing a discharge medium, an electrode arrangement with an anode and a cathode, and having a dielectric layer between at least the anode and the discharge medium, the electrode arrangement being inhomogeneous along a control length in a way which varies a burning voltage, by virtue of the fact that it defines along the control length a discharge spacing which varies monotonically at least in a local mean value, and it holds for the quantitative ratio between a difference between a maximum arcing distance dmax between the electrodes in the control length and a minimum arcing distance dmin between the electrodes in the control length and this control length that: (dmaxxe2x88x92dmin)/SLxe2x89xa60.6, and an electric parameter of the power supply of the discharge lamp is varied during operation in order to control the power of the discharge lamp.
Furthermore, the invention also relates to a lighting system having the discharge lamp described and having a ballast designed for the method just mentioned.
Preferred design variants relating to the operating method according to the invention and to the lighting system according to the invention are specified in the dependent claims.
Some of these refinements of the invention are also associated with further technical features of the discharge lamp. To this extent, the invention likewise relates to the correspondingly configured discharge lamp.
As is already to be gathered from the preceding general formulation of the invention, the invention is directed toward power control in discharge lamps with dielectrically impeded discharges. It provides for this purpose at least one control length along the course of the electrode in the discharge lamp. This term denotes a segment of the electrode structure along which inhomogeneous discharge conditions exist. The aim of this inhomogeneity in the discharge preconditions is for a burning voltage of the discharge to vary monotonically along the control length, but at least to vary monotonically in an effective mean value. A particular discontinuous possibility for monotonic variation in the burning voltage is still to be examined further below.
In this case, the term burning voltage relates, in particular, to a minimum burning voltage which corresponds not to the starting voltage of an individual discharge, but to the minimum voltage with which a discharge structure can be maintained at a specific point of the electrode arrangement.
In the case of this invention, it is preferred to consider an operating method in which the active power is injected into the discharge lamp in a pulsed way. Reference is made for this purpose to WO 94/23442 and DE-P 43 11 197.1.
The disclosure content of these applications is hereby also referred to.
In conjunction with this pulsed active-power injection, in this case restarting does not mean restarting of an individual discharge in conjunction with a still remaining residual ionization after one of the regular interruptions or dead times of the active-power injection, which occur in continuous lighting operation in accordance with the pulse principle. Rather, the starting voltage required for restarting means the situation in which the discharge lamp is switched on entirely from new, that is to say without residual ionization still being present in the discharge medium.
A property of discharge lamps for dielectrically impeded discharges which is important in connection with this invention is the positive current-voltage characteristic. Owing to the unambiguous relationship between current and voltage in this characteristic, it is thereby possible by varying the supply voltage also to vary the lamp current by means of the dielectrically impeded discharges. A negative differential resistance opposes this in the case of conventional discharge lamps.
The invention is based on the following observation in conjunction with this variation in the lamp current. A substantial advantage of the pulsed mode of operation referred to here resides in the fact that the dielectric impediment is utilized favorably to such an extent that discharge structures are produced having a shape which is relatively widely fanned out in front of the impeding dielectric. In these typical discharge structures, relatively low charge carrier concentrations, which are of very great significance for the efficiency of the discharge lamp operation, prevail, at least for the predominant portion.
Consequently, in the case of conventional structures lamp current rises are associated directly with an increase in the charge carrier concentrations in the individual discharge structures, and thereby worsen the efficiency of the light production.
Furthermore, excessively high lamp currents lead to a substantial thermal loading on the cathodes (or instantaneous cathodes in the case of bipolar operation), for which the discharge structures exhibit relatively concentrated attachment points. Consequently, the relative cathode points are subjected to punctiform thermal loading. Moreover, an amplified lamp current also increases the erosion effect owing to the ion bombardment at the cathodes, that is to say the sputtering effect of the discharges.
On the other hand, however, disadvantages are also associated with allowing the lamp current to drop below an optimum value, because instabilities can then occur, and individual charge structures can be extinguished or jump back and forth between different points. The spatial and temporal homogeneity of the light production is worsened thereby.
If, in a conventional way, the lamp current is increased beyond an optimum value or drops below this optimum value, this is associated in each case with substantial disadvantages. The invention proceeds from the basic idea of increasing the current in the discharge lamp by varying the total volume of the discharges such that the current density can remain substantially the same in the individual discharge structures. This volumetric change in the discharges can be produced within the control length in basically two different ways. In one case, a single discharge structure is enlarged to form a discharge structure which is widely extended like a curtain. In the other case, a plurality of partial discharge structures are juxtaposed within the control length, such that a variation in the number of these partial discharge structures within the control length varies the total volume of the discharges. The transition between the two cases outlined can also be fluid under some circumstances.
In any case, at least on the anode the discharge structures stretch over a finite range of length along which the discharge preconditions change in the sense of the spatially dependent burning voltage according to the invention. It is possible here for the case of the juxtaposed individual discharge structures to imagine in each case a local averaging for each discharge structure such that the mean values reflect the spatial dependence of the discharge structures. In the case of a discharge structure which widens like a curtain, the spatial dependence of the discharge preconditions is responsible for the fact that the corresponding limit of the discharge structure can be displaced along the electrodes within the control length.
If the spatial homogeneity of the light production plays a substantial role in the discharge lamp, the control length can thus be of relatively small dimension by comparison with the overall size of the discharge lamp, that is to say the discharge lamp can be divided into a plurality of individual control lengths. A variation in the discharge volume within the individual control lengths can then be compensated in a suitable way by averaging the light production, for example by diffusers, prismatic foils or the like. This results overall in a homogeneous character of the light production, there being no need for the power variation owing to an increase or decrease in the currentxe2x80x94for example, as a consequence of an increase or decrease in the voltage injectionxe2x80x94to be associated with a clearly visible variation in the discharge structures.
There are various possibilities for such an inhomogeneous electrode arrangement for a monotonic spatial dependence of the minimum burning voltage within the control length. What is most important and provided in any case according to the invention consists in a variation in the spacing governing the discharge, the so-called arcing distance, between the electrodes. The larger the arcing distance, the higher is the minimum burning voltage for a discharge over this spacing.
To this extent, the invention is thus directed to an electrode arrangement in which the arcing distance is varied monotonically along the control length, at least in a local mean value.
Moreover, within the scope of the invention a quantitative limitation holds for the relationship between the fluctuations in the arcing distance, that is to say the difference between the maximum arcing distance dmax occurring within a control length and minimum arcing distance din and the control length SL itself as path length. The upper limit for this ratio is 0.6, preferably 0.5. The value 0.4 is particularly preferred here.
The ratio just described can also assume very small values within the scope of the invention, as long as it does not vanish. Perceptible effects of the invention can be achieved starting from values as low as 0.01, for example.
In this context, it is possible to explain the difference between the already mentioned starting voltage and the minimum burning voltage to the extent that a discharge at a specific point on the control length with the monotonically varying electrode spacing can certainly ignite an adjacent region with a small spacing and then migrate into the region in which the instantaneous available burning voltage is precisely still sufficient for the discharge. This goes back to the basic phenomenon that the discharge structures are distributed if possible over the available electrode surfaces, because local space charges build up which increasingly shield the electric field in the discharge medium and widen the discharge structure by influencing the field distribution.
However, it is also perfectly possible in the case of the invention for the electrodes to be provided with points (already known per se) for spatial field forcing and thus for localizing individual discharges. It is not directly possible in the case of such structures to move individual discharge structures between these points with a discharge spacing which is sufficiently short in each case to ignite a discharge, and other points at which the spacing only still suffices to maintain a discharge. Specifically, it can happen that the region between the points of local field forcing are even no longer capable of maintaining the discharge.
In the context, discussed here, of the arcing distance or the discharge spacing as determining variable for the burning voltage, such local field forcings can be caused, for example, by small projections or lungs on one or both electrodes. The determining discharge spacing is then measured from the respective tip of such a projection. This means it is possible in this connection for there to be a discontinuous sequence of burning voltages at the respective points, in which case the invention is preferably directed to the case in which these points of local field forcing define a monotonically graded sequence of different burning voltages within the control length.
It may be shown in this case that the burning voltage named in claim 1 can also correspond to the starting voltage for a discharge and not to the minimum burning voltage for maintaining it. Of course, transitions between these extreme cases are also conceivable in the case of the invention. In this sense, the term burning voltage must be understood as being adapted to the respective situation of the electrode arrangement.
In addition to the variation, just discussed, of the discharge spacing for the purpose of influencing the burning voltage, an additional possibility consists in varying the anode width. Firstly, the anode width determines the local anode surface available for the discharge, and thus the discharge current. Again, the discharge current determines the residual ionization of the discharge medium which remains at the end of a dead time interval between two active-power pulses and which determines the probability of restarting and also the restarting voltage. In addition, in the case of a relatively large anode surface, and thus a distribution of the discharge current over a larger surface, there is a smaller voltage drop across the dielectric, and thus a larger electric field in the discharge medium.
Of course, a variation in the anode width can also occur here in conjunction with the described cathode projections, and does not necessarily presuppose substantially smooth cathodes.
Finally, there is also the further possibility of varying the thickness of the dielectric in order, in a way resembling the previous explanation, to influence the discharge current and thus the electric field in the gas filling. An inhomogeneity in the electrode structure can also in this way cause a local variation in a burning voltage of the discharges.
Thus, it is possible in the case of the invention on the one hand to provide a controllable number of individual discharges within a control length, or to influence the individual volumetric extent of a discharge structure respectively assigned to a control length. In the last case, the invention relates to a curtain-like spreading of a discharge structure in the control length by means of a suitable electrode structure with a monotonically spatially dependent burning voltage.
Variants of the invention with a continuous profile of the burning voltage along the control length and with a spatial dependence which is, rather, discontinuous have been explained above. The term of power control is therefore to be understood in general terms as regards the invention. Thus, it can certainly refer to switching the discharge lamp between different discrete power levels, it being possible to prescribe the power levels on the one hand by means of the already described discontinuous electrode structures with points of local field forcing with respectively assigned individual discharges, and also by means of electric levels of a corresponding ballast.
However, the invention is preferably directed to a dimming circuit for a discharge lamp with dielectrically impeded discharges. The term xe2x80x9cdimmingxe2x80x9d in this case means a power control in the case of which a specific dimming range can be traversed in a continuous way, or in an at least approximately continuous way, by the power control. For the case described of a xe2x80x9cdiscontinuous solutionxe2x80x9d, this means that a relatively large number of points of local field forcing must be present within the control lengths, in order to be able to undertake an at least approximately continuous adjustment of the power within this selection of power levels.
So far, control by the voltage at the discharge lamp has been spoken of by way of example in connection with the adjustment of the discharge current and of the discharge volume. However, the invention is to be understood more generally; it is basically an xe2x80x9celectric parameterxe2x80x9d that is spoken of for adjusting or controlling the power. In this case, in addition to the voltage present across the discharge lamp the following variants come into consideration as regards the pulsed active-power injection:
firstly, the steepness of an edge rise can be influenced in the case of the pulsed active-power injection. This variant relates to a certain extent to the time derivative of the voltage present across the lamp in the region of the rise of the individual pulse. This concerns, firstly, an empirical result of the development work on which this invention is based. A possible explanation of this control option consists, however, in that given a steeper voltage rise, and thus given a stronger participation of radio-frequency Fourier components in the voltage profile, the radio-frequency conductivity of the dielectric, in particular, is improved by comparison with a low-frequency or dc conductivity, and thus the electric field existing in the gas filling is increased, as already explained in another context. Furthermore, a role is played here by a variation in the electron energy distribution by the time derivative of the electric field.
A further time parameter of the active-power supply for influencing the burning voltage in the discharge lamp is what is known as the dead time between the individual active-power pulses, that is to say the time in which no discharge burns between individual pulses. The longer this dead time proves to be, the lower, of course, is the residual ionization remaining in the discharge medium at the end of the dead time. Again, add the probability of restarting or of the voltage required for restarting, depends on the extent of the residual ionization.
Finally, as further temporal parameters of the active-power supply mention remains to be made of the pulse duration and the repetition frequency of the pulses, which can be used in accordance with this invention in a similar way as previously explained in relation to the control of the power.
In order to vary the discharge spacing, it is preferred according to the invention to operate in the region of the continuous variations in the discharge spacing with a sinusoidal shape, at least of one of the electrodes, or with a sawtooth shape of at least one of the electrodes. The sinusoidal shape is formed in a fashion free from tips, that is to say round throughout. Such tips can lead to local field forcing. This can be undesired in some cases. On the one hand, the field forcings can facilitate initial starting. On the other hand, they leadxe2x80x94on one anodexe2x80x94to increased current densities, and can thereby worsen the efficiency of the discharge.
Furthermore, the sinusoidal shape has the advantage that it runs symmetrically to two sides starting from an extreme value, that is to say permits the discharge structure to be drawn open simultaneously in two directions like a curtain. In this case, the centroid of the discharge structure remains constant, in particular, and this can be advantageous with regard to the external appearance of the discharge lamp.
Again, the sawtooth shape can also, of course, be rounded with regard to the tip of the sawtooth which has just been addressed as a possible disadvantage. It can also be bilaterally symmetrical, or else asymmetrical, that is to say the sawtooth shape comprises, for example, a short steep ramp and a long but less steep ramp. An essential point of the sawtooth shape is the linearity of the ramp, that is to say the linearity of the spatial dependence of the discharge spacing. It follows that there are largely identical conditions over the control length between the external intervention in an electric parameter and the resulting spread of the discharge structurexe2x80x94aside from the precise mathematical relationship between the changed electric parameter and the discharge spacing.
However, it can also be precisely desired not to design the tip of a sawtooth shape as rounded. The local field forcing, already addressed, therefore creates in front of a tip directed toward the corresponding counter-electrode a situation which facilitates the initial starting of a discharge. Nevertheless, it remains possible to draw open a discharge structure like a curtain starting from this tip. A corresponding statement also holds for a juxtaposition of a plurality of individual discharge structures within the control length.
A further preferred quantitative relationship between the minimum arcing distance don and the maximum arcing distance dmax within the same control length can be specified as follows. A ratio of the minimum arcing distance to the maximum arcing distance of more than 0.3, preferably 0.4 and 0.5, as well as below 0.9 is favorable.
In conjunction with the definition of the control length, it is important to mention that the control length need not necessarily correspond to the maximum possible distance between a minimum electrode spacing described by the geometric electrode structure and a maximum electrode spacing. Consequently, control length means the length of the electrode arrangement actually utilized by the power control according to the invention.
This distinction is important chiefly in the case of electrode structures, for example sinusoidal or sawtooth shapes already addressed, which xe2x80x9ccan be usedxe2x80x9d starting from two different sides. Specifically, in the case of a strip arrangement, preferably taken into consideration here, of electrodes on a wall or on opposite walls of a discharge vessel, an alternating sequence of electrodes can be present in such a way that at least some of the electrodes are used for discharges to two sides, in particular to opposite sides. Since the discharges burning to the two sides interfere with one another on the electrode strip, it is possible here, for example in the case of a sinusoidal shape, for a specific part of the sine to be assigned to one possible discharge side, and for another part to be assigned to the other possible discharge side, generally the respectively immediately adjacent part, of course. In particular, it is also possible in this case to provide a certain intermediate section between the regions respectively assigned to other discharge sides, starting from which fundamentally no discharges are to emanate.
With regard to the drawing open of the width of a discharge structure in accordance with the invention, it has proved to be important that any layers situated on the electrodes, in particular on the cathode, are relatively smooth. Troublesome instances of graininess can occur, particularly in the case of phosphors, which are usually deposited in a relatively two-dimensional fashion using the printing method, and can therefore certainly also lie on the electrodes. A sensible quantitative limit is a graininess of 8 xcexcm, starting from which downward it is possible to open out the width of a discharge structure on such a layer. Of course, instances of smaller graininess of 5, 3 or 1 xcexcm and less are more suitable. It is to be assumed that the graininess constitutes a basic problem of all layers, and to that extent is not limited to phosphor layers. On the other hand, given the present state of the art the phosphor layers are, in particular, occasionally relatively coarse grained. If, for specific reasons, there is no sufficiently fine grained alternative to a phosphor layer, it is preferred in this case in accordance with the invention to leave the cathode completely free from phosphor, that is to say to omit the deposition of the phosphor. Other layers, for example fine grained reflecting layers made from TiO2 or Al2O3, are not necessarily affected thereby.
These statements are not, however, to be understood to the effect that the method according to the invention would not be functional with a grainy phosphor layer or another grainy layer on a cathode. Yet further parameters play a role here, for example, the steepness of the rise in the discharge spacing over the control length, and these can be used to permit appropriate drawing open even in the case of grainy layers.
In a preferred variant of the operating method according to the invention, a lamp is driven with the aid of bipolar voltage pulses, that is to say a voltage pulse generated by the ballast is followed by a voltage pulse of inverse sign (polarity). Here, the lamp has a two-sided dielectric impediment, that is to say all the electrodes are covered with a dielectric layer. The bipolar operating method is suitable, in particular, for the electrodes described here which are of the same type from the point of view of discharge physics and can take over in a temporally alternating fashion the role both of a temporary anode and of a temporary cathode.
An advantage of the bipolar operating method can reside, for example, in rendering the discharge conditions in the lamp symmetrical. Problems caused by asymmetrical discharge relationships can thereby be avoided particularly effectively, for example, ion migrations in the dielectric, which can lead to blackening, or to space charge accumulations which worsen the efficiency of the discharge.
A modified forward converter, for example, comes into consideration as ballast for the bipolar operating mode. The modifications aim at providing for a reversal of direction in the primary-side current, which effects the voltage pulse in the secondary circuit, in the transformer of the forward converter. This is generally simpler than making corresponding electrotechnical measures to reverse direction on the secondary side.
In particular, for this purpose the transformer can have two primary-side windings which are assigned in each case to one of the two current directions, that is to say only one of the two directions is used for a primary circuit current. This means that current is applied in an alternating fashion to the two primary-side windings. For example, this can be performed by using two clocking switches in the primary circuit, which respectively clock the current by an assigned one of the two windings. Each of the two current directions is thereby assigned a dedicated clock switch and a dedicated primary-side winding of the transformer.
When a ballast according to the invention is used on an ac source, it can be advantageous with regard to the two primary-side current directions to use two storage capacitors which are charged alternately by half period from the ac source. Thus, the ac half periods of one sign are used for one of the storage capacitors, and the ac half periods of the other sign are used for the other storage capacitor. The currents of one direction in each case can then be extracted from these two storage capacitors. This can be performed together with the outlined dual design of the primary-side winding of the transformer, but such a design is not actually required here. However, a single primary-side winding can be supplied in alternating fashion from the two storage capacitors by corresponding switches, each storage capacitor respectively being assigned to one current direction. In order to feed the storage capacitors from the ac source, use may be made of an appropriate rectifier circuit whose details are immediately clear to the person skilled in the art.
As already stated, the invention is directed not only to an operating method for a corresponding discharge lamp, but also to a lighting system, which denotes a suitable set comprising a discharge lamp and a ballast. In this case, the ballast is designed with regard to the method according to the invention, that is to say the ballast has a power control device with the aid of which a suitable electric parameter of the power supply of the discharge lamp can be influenced by the ballast in order to make use of the appropriately configured discharge structure in the discharge lamp to vary the discharge volume.
To this extent, the above statements relating to the various refinements of the invention also apply likewise to the lighting system, that is to say in each case to the electrode structure in the discharge lamp and to the power control device in the ballast.
With regard to the particular features of the electrode structure explained in the previous description, protection is also claimed for a correspondingly configured discharge lamp, reference being made for this purpose to the corresponding explanations in the previous description.